Ion implantation has been used in the manufacture of many types of semiconductor devices. For example, ion implantation has been used for adjusting the threshold of the load device in an MOS inverter circuit. However, when such a process step was used the following results were obtained which are substantially different from the results of the present process.
In such an environment the adjustment of the load devices would cause the load devices to operate in the depletion mode. This means that the threshold voltages would go below zero in the case of N-channel silicon gate devices. In our situation, the devices do not adjust to the point where they go depletion but actually the threshold voltages are increased so they actually operate in the opposite manner. Ion implantation has also been used to adjust the threshold voltage for P-channel devices in order to lower the threshold voltage such that the P-channel metal gate devices would be compatible with T.sup.2 L circuits. This is contrasted with the present process which is using the ion implantation to adjust upwards the threshold voltage.
A competing process to the present invention utilizes a starting substrate of relatively low resistivity material, i.e. the resistivity lying within the range of 1 to 3 ohms centimeter material. However, when using the relatively low resistivity starting material and connecting the completed devices to one power supply, the output signal available from such a device is in the 2.8 to 3 volt range. This is compared to the process wherein the starting material is a relatively high resistivity material lying within the range of 10-45 ohm centimeter and the gate regions of both the switch and the load devices are given an ion implant according to the dosage and concentrations as set out hereinafter. This device made by this process gives a substantially lower body effect resulting in a substantially higher output voltage lying within the range of approximately 4 volts when using a 5 volt power supply. Essentially this is a 25% improvement over the prior art process.
In review, ion implantation in the prior art has been used to lower the threshold voltage on P-channel devices as well as to make the N-channel devices operate in a depletion load. Additionally, the prior art process using a 5 volt single power supply has resulted in high body effects causing adverse condition of the output voltage such that the maximum voltage obtainable lies within the range of 2.8 to 3 volts. The present process has caused the threshold voltage to be adjusted upward which is exactly opposite from the prior art thinking and hence an unobvious result and has effectively reduced the body effect of the structure on which the process has been practiced thereby resulting in a higher output voltage of about 4 volts using a 5 volt power supply.